Switchable resonant filter for optical radiation

ABSTRACT

A structure to be used as a switchable optical filter is disclosed. The structure comprises a substrate transparent to the radiation of interest, which contains or supports an electrode. A spacer layer suspends the membrane over the electrode such that no spacer lies between the electrode and the membrane, yet some spacer remains elsewhere to support the membrane. The membrane has slots in it which are of a specified length in order to resonate at a particular wavelength of radiation. When the electrode is activated, the membrane deflects and its varying proximity to the substrate changes the wavelength of transmission. The method of manufacture is also disclosed.

BACKGROUND OF THE INVENTION

1. Field of the invention

This invention relates to optical sensors, more specifically to filtersfor said sensors.

2. Background of the invention

Optical sensing systems, either in the visible range, or thenon-visible, such as infrared, can fall victim to jamming effects. Invisible systems, this is normally referred to as veiling. Veilingilluminance is defined as light that is spread across a large portion ofthe field of view, lessening system effectiveness by lowering scenecontrast. Common sources of veiling are sunlight and high-intensitysources. Jamming can also be caused deliberately, such as in defenseapplications, when an enemy electro-optic source is aimed at the subjectsystem.

A number of sensors currently in use contain fixed stop-bandinterference filters in slides or wheels for protection from theseproblems. This approach supplies a limited number of stop-band filterchoices and requires operating mechanisms with control circuitry,whether they are mechanical or electromechanical. Activation of thesefilters takes fractions of seconds, not including operator responsetime. This fixed-filter approach works well for single-wavelength,non-agile threats. However, fixed-filter techniques are limited againstthe agile threats expected on the modern battlefield.

Another weakness of the fixed-filter system becomes apparent whenconsidering bandwidth. In the 8 to 12 micron (10⁻⁶ meter, or μm)wavelength range, it is not practical to use filters having a bandwidthless than 1 micron. For an infrared system, such as a Forward-LookingInfraRed (FLIR), the integrated transmission of a filter with a single 1micron stop-band is no better than 50%. Narrower-band filters tend to bemore expensive and have insufficient integrated transmission.

Another area of concern with current technology is the difficulty ofautomation. The system detector can be used to sense when a filter isneeded. However, the sensor cannot detect when the filter is no longerneeded, since the filter prevents the sensor from seeing the threat. Thesensor could be damaged if the filter is switched out and the threat isstill present. The long cycle times of mechanical filter assembliesincrease the likelihood of damage, if the threat is still present.

SUMMARY OF THE INVENTION

The present invention disclosed herein comprises a switchable resonantfilter for optical radiation. One embodiment of the present inventioncomprises a membrane monolithically manufactured on substrate coatedwith anti-reflective layers, and containing electrodes. The membrane isformed upon a spacer layer which is laid over the substrate andelectrodes, and portions of the spacer layer over the electrodes areremoved, via holes etched in the membrane. The holes in the membrane actas antennas for radiation that has the same wavelength as the electricallength of the holes. The electrodes can be addressed to pull themembrane down to the substrate, changing the effective index ofrefraction surrounding the antennas, and thus change the resonantwavelength of the antennas. The preferred embodiment of the presentinvention acts as a switch for transmission of radiation at specificwavelengths through the membrane and substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and forfurther advantages thereof, reference is now made to the followingDetailed Description taken in conjunction with the accompanying Drawingsin which:

FIG. 1A shows a side view of a switchable filter membrane device withthe membrane undeflected.

FIG. 1B shows a side view of a switchable filter membrane device whenthe membrane is deflected.

FIG. 2A shows a flow chart of the manufacturing process flow for aswitchable resonant filter for optical radiation.

FIG. 3 shows the dimensions of a possible geometry of a resonant slot tobe made in a membrane.

FIG. 4 shows an alternate embodiment wherein the membrane is transparentand includes conducting members.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is well known that a wire can act as an antenna for certainwavelengths of electromagnetic radiation. The length of the wire as wellas the index of refraction, or the dielectric constant, of thesurrounding medium, determines the wavelength of the radiation itabsorbs. The index of refraction of the medium determines the wavelengthof radiation having a given frequency. If the antenna is embedded in ahigher index medium, for example, the antenna would have to be shorterto absorb waves of a certain frequency. If made small enough, theantennas will absorb extremely high frequency waves such as infrared(IR).

Consider a fixed wire or antenna illuminated by a variety ofwavelengths. If the effective length of the antenna were l_(eff), theantennas would absorb wavelengths, λ, for which l_(eff) equals integralmultiples of one-half λ. The absorption is caused by losses due to thefinite conductivity of the metal.

Instead of a wire, it is possible to make something similar from a sheetof metal with slots the same length as the antenna in the sheet ofmetal. The behavior of this structure is such that it reflects allwavelengths because of the sheet of metal, except those that areresonant at the length of the slots. Therefore, all wavelengths exceptfor the selected one are stopped. Radiation of the selected wavelengthis passed, and the structure transmits it. The result is a filterselectively passing wavelengths to which these slots are tuned. Oneparticular application is in the infrared range, but it could be usedfor any wavelength of radiation, limited only by the practicality offabricating slots with the required dimensions.

If the surrounding medium is changed, that will change the wavelength towhich the slots are resonant. In order to cause this change, theeffective dielectric constant around the slot or slots must be changed.This effectively switches the wavelength that the slots will transmit.

One embodiment of the present invention is shown as a side view in FIGS.1A and 1B. A substrate 10, contains an electrode, 12, and can be coatedwith antireflection coatings 14, or another insulating coating. A spacermaterial 16 lies on top of the coating, if used, or the electrode. Thespacer 16 must be selectively removable, such as with an etch. A thinmetal membrane 22 rests upon the spacer. The membrane 22 has slots 24patterned into it, which also act as etch access holes to remove thespacer. The materials of these various components of this structure willbe discussed further as part of the manufacturing process.

FIG. 1B shows the membrane 22 in contact with the substrate or itscoating. This is caused by activation of the electrode 12. When a chargeis placed upon electrode 12, by addressing circuitry not shown,electrostatic forces build between the electrode and the membrane,attracting the membrane to the electrode. The membrane will eventuallycome to rest on the substrate. If the anti-reflection coatings are notused, some kind of insulating layer is necessary to prevent shortingbetween the electrode and the membrane.

The movement of the membrane causes the effective refractive index ofthe medium surrounding the slots to change. If the slots were initiallydesigned to transmit a wavelength λ, the initial transmitted wavelengthof radiation would be λ/n_(eff), where n_(eff) is approximately equal tothe refractive index of the surrounding gas or vacuum. When the membranecomes into contact with the substrate the transmitted wavelength changesto λ/n_(eff), where n_(eff) is in the range 1.0<n_(eff) <n_(substrate).If the membrane is in contact with a different medium on one side thanthe other, the effective index is a special kind of average of theindices of the two surrounding media: ##EQU1## where n₁ and n₂ are theindices of refraction for the medium on either side of the membrane.This change in the peak transmitted wavelength will allow the sensingsystem to discriminate against jamming, by reducing transmission of thejamming wavelengths.

FIG. 2 shows a flow chart for the manufacture of such a filter. Theinitial step 26 is to define an electrode, or electrodes, in thesubstrate. The substrate is preferably transparent to the wavelength ofinterest, such as silicon, or gallium arsenide, which is well-suited forthe 8-12 micron wavelength band. The electrode can be formed in manyways. The electrode could be formed by doping, either blanket orpatterned. After an implant, a drive-in diffusion may be necessary toactivate the dopant and provide sufficiently low resistivity for theelectrode. Alternatively, a deposited film electrode, of a transparentconducting material, such as indium tin oxide, or a metal patternedtransversely in such a way as to minimally obscure the path of lightthrough the device, is possible. Further, the electrode could also bethe substrate, where the substrate is partially conductive. There is aninherent trade-off between the conductivity and transparency of thelower electrode.

Continuing in the process, the next step is step 28, which is thedeposition of the spacer layer. If anti-reflection coatings are to beused, the "top" layer, the one between the electrode and the spacerlayer, must be applied before the spacer is deposited. For example, theycould be deposited by evaporation. The effective index seen by the slotswill then be influenced more by the coating index, than by the substrateindex. According to the preferred embodiment of the present invention,the spacer comprises a polymer layer, applied by spinning.

The major requirement of the structure is that in its unactuated state,the membrane is far enough away from the substrate such that thesubstrate does not affect the wavelength that is absorbed or transmittedby the slot. This distance can be less than a wavelength. The closer themembrane is to the substrate, the more critical the distance betweenthem becomes.

The thickness of the spacer must be such that when it is removed, themembrane will be held sufficiently far from the substrate so that theeffective refractive index seen by the membrane is significantlydifferent from the index experienced when it approaches or touches thesubstrate. The effective index of the surrounding medium can be that ofair, which is approximately 1.0, if the membrane is far enough away onboth sides from any other medium.

In step 30, the membrane is deposited, most likely by sputtering. Themembrane is a thin, tensile film of a reflective metal such as gold oraluminum alloy. After the membrane is deposited, it is patterned andetched in step 32 to form the tiny access holes for the etch, as well asthe slots. It is possible that the slots will suffice as the etch accessholes.

An isotropic selective etch, such as a plasma etch, is used in step 34to remove the spacer over the electrodes, under the slots, and leavespacer around the edges to support the membrane. Depending on the etchprocess selected, it may be beneficial to dice the wafer upon which thestructure has been constructed before etching. More than one such devicemay be manufactured upon one wafer. The amount and position of remainingspacer is controlled by the extent of the access holes in the membrane,and the time of the etch. The final result is a thin metal membranecontaining a resonant slot pattern supported over an air space where thespacer has been removed. An alternate embodiment could involve multipleelectrodes under a single membrane. The spacer would then be left in agrid-like pattern, allowing each electrode to control a defined area ofthe membrane. The membrane can be electrostatically deflected byapplication of a voltage between the membrane and the underlyingelectrode. After the completed structure is finished, it must bepackaged and placed in the sensing system, as in step 36. The devicewill typically be held by the edges, somewhere in the sensing module.The electrical connections are normally best done from the substrateside of the device. Protective packaging may be necessary, dependingupon the operating environment of the system.

One problem with using a straight-line slot is that it transmits onlyradiation whose electric field vector is aligned with the direction ofthe slot. One solution is to use a pattern of slots oriented in avariety of directions. However, the efficiency of the maximumtransmission for a given polarization will be limited by the fraction ofproperly-oriented slots. To alleviate this problem, it is possible tocreate a polarization-insensitive device by using a cross or tripole. Inthe tripole, the slot is designed such that there are three legs, each120° degrees from the two adjacent legs.

The geometry of a tripole slot is shown in FIG. 3. In the case of atripole, the size is determined by three factors: the freespace opticalwavelength of the radiation of interest, λ_(O) ; the effectiverefractive index of the surrounding medium, n_(eff) ; and, a geometricalfactor, K. The geometrical factor for a tripole equals≈0.27. The angle40 is 120°. The length of a leg is measured from its end, 42A or 42B, tothe center of the joint, 44. This distance, L_(T), is shown by line 46.The length, L_(T), at which the first resonance of a tripole occurs isgiven by the formula: ##EQU2##

When the device is in operation and the membrane contacts the substrate,the center wavelength of the transmitted band is shifted. The width ofthe transmitted wavelength band is affected, among other things, by theuniformity of the sizes of the slots, as determined by fabricationparameters. The transmission wavelength is longer than before, allowinga switch between two optical bands, or an ON/OFF switch for certainwavelengths. Additionally, it could be used as a radiation chopper, asone of many alternatives.

An alternate embodiment of the present invention could be to use atransparent membrane with a number of conducting members in it. This isshown in FIG. 4. In FIG. 4, the membrane 22 is transparent. The membranehas within it, or on one of its surfaces, conducting members 48. Theseare shown as tripoles, but could be of any geometry desired, limitedonly by manufacturing concerns.

Thus, although there has been described to this point particularembodiments of a switchable resonant filter for optical radiation, it isnot intended that such specific references be considered as limitationsupon the scope of this invention except in-so-far as set forth in thefollowing claims.

What is claimed is:
 1. An optical filter comprising:a. a substratesubstantially transparent to a predetermined wavelength of opticalradiation; b. at least one electrode formed proximate said substrate; c.a spacer formed upon a layer containing said electrode such that saidspacer does not cover said electrode; d. a membrane formed upon saidspacer such that said membrane is suspended over said electrode,supported by said spacer; and e. slots formed in said membrane such thatsaid slots are resonant for optical radiation of said predeterminedwavelength.
 2. The filter of claim 1 wherein said substrate furthercomprises gallium arsenide.
 3. The filter of claim 1 wherein saidsubstrate further comprises silicon.
 4. The filter of claim 1 whereinsaid electrode further comprises a partially conductive substrate. 5.The filter of claim 1 wherein said electrode further comprises a dopedregion in the substrate.
 6. The filter of claim 1 wherein said electrodefurther comprises a film electrode of a transparent conducting material.7. The filter of claim 6 wherein said transparent conducting material isindium tin oxide.
 8. The filter of claim 1 wherein said electrodefurther comprises metal patterned transversely so as to minimallyobscure the path of light.
 9. The filter of claim 1 wherein saidmembrane is of aluminum alloy.
 10. The filter of claim 1 wherein saidmembrane is of gold.
 11. An optical filter comprising:a. a substratesubstantially transparent to a predetermined wavelength of opticalradiation which has two parallel faces; b. at least one electrode formedproximate one of said two parallel faces; c. anti-reflection coatingsformed upon the substrate on at least one of said two parallel faces; d.a spacer formed upon a layer containing said electrode such that saidspacer does not cover said electrode; e. a membrane formed upon saidspacer such that said membrane is suspended over said electrode,supported by said spacer; f. slots formed in said membrane such thatsaid slots are resonant for optical radiation of a predeterminedwavelength.
 12. The filter of claim 11 wherein said substrate furthercomprises gallium arsenide.
 13. The filter of claim 11 wherein saidsubstrate further comprises silicon.
 14. The filter of claim 11 whereinsaid electrode further comprises a partially conductive substrate. 15.The filter of claim 11 wherein said electrode further comprises a dopedregion in the substrate.
 16. The filter of claim 11 wherein saidelectrode further comprises a film electrode of a transparent conductingmaterial.
 17. The filter of claim 16 wherein said transparent conductingmaterial is indium tin oxide.
 18. The filter of claim 11 wherein saidelectrode further comprises metal patterned transversely so as tominimally obscure the path of light.
 19. The filter of claim 11 whereinsaid membrane is of aluminum alloy.
 20. The filter of claim 11 whereinsaid membrane is of gold.
 21. A method of forming an optical filtercomprising:a. defining an electrode proximate a substrate; b. depositinga layer of spacer material upon said electrode; c. forming a membraneover said spacer; d. etching plasma access holes in said membrane,wherein said access holes comprise slots which are resonant at a certainwavelength of optical radiation; and e. removing said spacer materialsuch that said material is not present over said electrode, but remainselsewhere to support said membrane, such that said membrane is operableto deflect towards said substrate when said electrode is activated. 22.The method of claim 21 wherein said defining step further comprisesdoping the substrate to form the electrode.
 23. The method of claim 21wherein said defining step further comprises depositing a filmelectrode.
 24. The method of claim 23 wherein said film electrode is atransparent conducting material.
 25. The method of claim 24 wherein saidtransparent conducting material is indium tin oxide.
 26. The method ofclaim 23 wherein said film electrode is metal patterned transversely soas to minimally obscure the path of light.
 27. The method of claim 21wherein said depositing step further comprises depositinganti-reflection coatings upon said substrate.
 28. The method of claim 21wherein said forming step further comprises sputtering said membraneupon said spacer.
 29. The method of claim 21 wherein said removing stepfurther comprises a plasma etch.
 30. A method for optical filteringcomprising:a. directing optical radiation toward a membrane including atleast one slot which is resonant for a selected predetermined wavelengthof said optical radiation; b. activating an electrode proximate asubstrate having a selected refractive index, said electrode forattracting said membrane; and c. moving said membrane including saidslot toward said substrate to change the wavelength of said opticalradiation at which said slot resonates.
 31. An optical filtercomprising:a. a substrate substantially transparent to a predeterminedwavelength of optical radiation; b. at least one electrode formedproximate said substrate; c. a spacer formed upon a layer containingsaid electrode such that said spacer does not cover said electrode; d. amembrane substantially transparent to said predetermined wavelengthformed upon said spacer such that said membrane is suspended over saidelectrode, supported by said spacer, wherein said membrane includesconducting members.